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AprilTags not detected when close to camera

asked 2022-10-26 03:48:43 -0500

PatoInso gravatar image

Hello,

I can detect AprilTags that are "far" from the camera, ie. small on the image and even a little blur, but the detection fails when the tags are close to the camera and when the image is clean and neat, what is counter-intuitive.

Is this expected ? How can I detect the tags at all scales ?

NB1: Interestingly, if I increase the "quad-decimate" param form default 1.0 to e.g. 16.0 (to reduce the image resolution), I can detect the "close" tag but not the "far" one (seems logical to not detect the small).

NB2: I use the VISP C++ lib directly, not the ROS apriltag packages, and images with 640x360 images

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answered 2022-10-26 04:09:48 -0500

nils_iseke gravatar image

updated 2022-10-26 04:10:38 -0500

Maybe you can get your anwser by reading through the paper, where the implementation is explained. Maybe one explanation is, that during the quad detection process only close enough lines are considered.

Robustness to occlusions and segmentation errors is handled by adjusting the “close enough” threshold: by making the threshold large, significant gaps around the edges can be handled. Our threshold for “close enough” is twice the length of the line plus five additional pixels. This is a large threshold which leads to a low false negative rate, but also results in a high false positive rate.

That would explain why reducing the resolution helps detecting the closer tag.

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Asked: 2022-10-26 03:48:43 -0500

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Last updated: Oct 26 '22